This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our group applies structural techniques to poorly understood mechanisms in signal transduction, such as light and temperature sensing, redox reactions involving long-range electron tunneling, and cooperative conformational changes within macromolecular assemblies. We have accordingly investigated signal propagation within large transmembrane protein complexes that mediate bacterial chemotaxis, pursued how circadian clocks are entrained by the environment, and studied the role of nitric oxide in bacterial pathogenesis and radiation resistance. Key to all of these projects is the crystallographic structure determination of proteins and their assemblies. In the chemotaxis system we are determining structures of chemoreceptors and their associated kinases, as well as receiver modules of the flagellar motor. In the circadian clock area with are focused on understanding the structure and mechanisms of light sensor proteins. In the nitric oxide area, we are determining structures of novel bacterial nitric oxide synthases in different activated states relevant to their catalytic cycle.